Looks like a fun day of skiing in one of my old home areas! 
In case of lots of fresh snow, many smaller ski areas are open as well. Altastenberg and Neuastenberg are for example good alternatives, when Winterberg gets overcrowded.
I agree that Paderborn-Lippstadt is the most convenient airport for a “fly to ski” fly-in event in that area. Fees are incredibly low for a full-service international airport, with the option of renting a rental car. Nonetheless, Meschede might be a good alternative for VFR during wintertime, when Paderborn is covered in low stratus clouds, while Meschede is in VMC. Brilon is not so convenient, with a hill on short final 07 and they still don’t allow unattended movements.
While I also didn’t fly this weekend, I was also on the ski slopes in Stoos on Saturday and sledding in Melchsee-Frutt on Sunday. Great conditions, even though it was extremely cold, especially on Sunday.
To get here by plane, the most convenient airfield would be Kägiswil (LSPG), as long as it exists for GA, and then continue with public transport to Melchsee-Frutt. It’s also a nice ski area, even though it’s not too big. The next IFR airport would be Grenchen (LSZG) or Zurich (LSZH). Stoos is also nice, but it does only have red or black slopes (except for the beginner area) and requires a rental car or taxi to get there from Buochs (LSZC) in a fast and convenient way. I wouldn’t recommend it for a “fly to ski” meeting.
I looked at Winterberg a few times some years ago but it seems sensitive to conditions, due to low altitude. Several trips were cancelled.
Yes, as I said, one needs to watch the conditions and be flexible. But conditions are quite stable right now. There will be a little more snow on Wednesday and then a lot of sunshine again next weekend. I might actually be going again.
